1. Field of the Invention
The present invention relates to a mixer that generates a multiplication signal using a magnetoresistive effect element and a frequency converting apparatus equipped with such mixer.
2. Description of the Related Art
A TMR (Tunnel MagnetoResistive) element constructed with a spacer layer formed of a nonmagnetic material between a fixed magnetic layer and a free magnetic layer is known as one example of a magnetoresistive effect element. In a TMR element, spin-polarized electrons flow when a current passes, and the magnetic orientation of the free magnetic layer (i.e., the orientation of an electronic spin) changes in accordance with the number of spin-polarized electrons that accumulate inside the free magnetic layer. When an attempt is made to change the magnetic orientation of a free magnetic layer that has been disposed in a certain magnetic field, torque will act so as to restore the electron spin to a stable orientation that is restricted by the magnetic field, and when the electron spin are perturbed with a specific force, oscillation referred to as “spin precession” will occur.
In recent years, a phenomenon called “spin torque ferromagnetic resonance” has been discovered whereby when a high-frequency AC current flows in a magnetoresistive effect element such as a TMR element, strong resonance will occur if the frequency of the AC current that flows through the free magnetic layer matches the oscillation frequency of the spin precession that is attempting to restore the magnetic orientation (see Tulapurkar, A. A. et al. 2005. Spin-torque diode effect in magnetic tunnel junctions. Nature 438: 339-342). It is also known that when an RF current (i.e., an RF current with a frequency that matches the oscillation frequency (resonant frequency) of the spin precession) is injected into a magnetoresistive effect element in a state where a static magnetic field is being applied to the magnetoresistive effect element from outside and the orientation of the static magnetic field is inclined by a predetermined angle in the fixed magnetic layer to the magnetic orientation of the fixed magnetic layer, the magnetoresistive effect element will function so that a DC voltage in proportion to the square of the amplitude of the injected RF current is generated across both ends of the magnetoresistive effect element, or in other words, achieve a square-law detection function (or the so-called “spin torque diode effect”). It is also known that the square-law detection output of such magnetoresistive effect element will exceed the square-law detection output of a semiconductor pn junction diode under predetermined conditions (see Suzuki, Y. et al. 2007. Microwave properties of spin injection devices: Spontaneous oscillation, spin-torque diode effect and magnetic noise. Magune 2(6): 282-290).